1. Field of the Invention
The present invention relates to telecommunications and radio frequency receivers, and more particularly to an image rejection mixer with switchable high or low side injection for a radio frequency to intermediate frequency converter.
2. Description of Related Art
The field of telecommunications has been developing rapidly. Hand-held mobile communications devices are now ubiquitous. With the popularity of such devices has come the demand for smaller size, higher speed, lower power consumption and lower cost. To accomplish these demands requires greater integration of the electronic components.
Most RF communications devices today use some variant of the conventional super-heterodyne approach. In conventional heterodyne receivers the received radio frequency (RF) is normally converted to one or more lower intermediate frequencies (IF) because the intermediate frequencies are more readily amplified and filtered. The RF signal is converted to an IF signal by injecting a mixing signal generated by a local oscillator into the RF signal. Unfortunately, the image of the RF signal can be mixed to the same IF frequency, which corrupts the desired output signal.
FIG. 1 is a graph showing low-side local oscillator (LO) injection, in which a carrier frequency (RF) is higher than a LO injection frequency and an image signal is lower than the LO frequency, while FIG. 2 is a graph showing high-side LO injection, in which the carrier frequency (RF) is lower than the LO injection frequency and an image signal is higher than the LO frequency. Filtering is necessary to attenuate such unwanted signals. If the unwanted signals are not attenuated sufficiently, they may be converted to a base band frequency that interferes with the desired output signal.
One method of attenuating such unwanted signals is via filtering. U.S. Pat. No. 6,097,269 assigned to Motorola, Inc., the assignee of the present application, discloses a single, tunable filter capable of both high and low side injection protection. The filter includes a pair of reactively coupled resonators, which are formed with varactors, capacitors and inductors.
FIG. 3 is a schematic block diagram of a conventional image rejection mixer 20 that provides one of either high-side injection or low-side injection. The image rejection mixer 20 includes a first mixer 22 that receives an RF input signal and a first local oscillator (LO) signal and generates a first intermediate frequency (IF) output signal. A second mixer 24 receives the RF input signal and a second LO signal and generates a second IF output signal. The first LO signal is a phase lag signal or sin ωlot and the second LO signal is a phase lead signal or cos ωlot, wherein ωlot is a frequency signal generated by a local oscillator (not shown). A phase shift circuit 26 is connected to the first mixer 22, receives the first IF output signal, and produces a phase shifted signal. The phase shifted signal and the second IF output signal are combined with a summer 28, thereby providing a final IF output signal.
If the phase shift circuit 26 generates a 90 degrees phase lead signal, the image rejection mixer 20 provides high-side injection. On the other hand, if the phase shift circuit 26 generates a 90 degrees phase lag signal, the image rejection mixer 20 provides low-side injection.
It would be advantageous if a low power image rejection mixer could be formed on a single substrate, without discrete components, that is switchable between high-side injection and low-side injection.